The present invention relates to a telecommunication carrier processor subsystem having an input and a plurality of outputs and being adapted to receive, at said input, telecommunication cells each comprising a payload field and a H-bit header field, said subsystem including telecommunication interface means having an interface input corresponding to said input and a plurality of outlets each coupled to distinct ones of said outputs, said telecommunication interface means including header detection means connected to said input and routing means connected to said input, to said plurality of outlets and controlled by said header detection means, said header detection means being adapted to derive a R-bit connection identifier from at least a portion of the set of H bits contained in said header field, R and H being integer numbers with R smaller than H.
In the frame of low earth orbit satellite constellations providing broadband local access solutions, the use of telecommunication cells is well suited for the transport of multimedia services. The cells permit to establish end-to-end connections between, e.g., a transmitting external telecommunication exchange or network coupled to the input and terminals. These connections are possible via earth stations comprising the telecommunication carrier processor subsystem, modems and terrestrial antenna stations. The cells are so transferred via satellites coupling the outlets of the telecommunication interface means to the terminals.
To transmit a cell from the input to a predetermined outlet, the routing means uses routing information that is generally extracted from the H bits contained in the header filed of the cell. However, the number of possible outlets and the number of connections that the terminals coupled to these outlets can handle represents only a part of the permitted number of connections that may be indicated by H bits. This limited part may for instance be represented by a set of R bits, where R is smaller than H. Due to other considerations, it is an unsuitable constraint to split the range indicated by the H bits into equal and contiguous fractions only to solve implementation problems. As a consequence, an interfacing device or header detection means is needed to derive the R bits of “useful” routing information from the H bits of “full” routing information contained in the header field of the cell.
Such a header detection means is already known in the art, e.g. from the European Patent Application EP-A1-0 862 348 entitled “Interfacing device to extract M sets of bits out of N sets of bits, control unit and logical cell”. As indicated in that document, the known header detection means or interfacing device is particularly useful when several data-handling units are connected to the outlets of a telecommunication interface means and different ones of these data-handling units can be addressed by only a portion of the data entering simultaneously at the input of the device. This is for instance so when the incoming sets of data bits are ATM [Asynchronous Transfer Mode] cells containing, in their header field, information of which only a portion is needed to further handle the cell. The known interfacing device may thus be used to reduce the number of H bits contained in the header field of a cell into a set of R bits sufficient to control the routing of the cell through the telecommunication interface means. According to the known document, such a set of R bits is then applied as a pointer to a RAM [Random Access Memory] of which the output is a connection identifier that controls the telecommunication interface means.
The output cell provided at an outlet of the telecommunication interface means is then either the former full cell (payload and header) received at the input but associated to the connection identifier issued from the RAM memory, or only the payload field of the former cell associated to this connection identifier. In any case, the length, i.e. the number of bits, of a cell at an outlet of the telecommunication interface means differs from the length of the former cell received at the input thereof.
Because the cells at the input generally have a standardized format, e.g. ATM cells, the design of specific apparatus able to manipulate non-standardized cell formats is required at the outlets of the telecommunication interface means. As a result, the development of a telecommunication carrier processor subsystem is relatively expensive and complex.